1. Field of the Invention
The present invention relates to a semiconductor memory device, and more particularly to a PROM (Programmable Read Only Memory) with a short data write-in time.
2. Description of the Prior Art
Recently, the PROM, which is one kind of a read-only memory has been widely used in computers and other data processors. Sub-routines and the like used in the data processor are written in the PROM in advance for use as a memory. Now, if the individual memory cells consist of floating gate type MOS transistors or the like, the memory contents can be erased, for instance by irradiation with ultra-violet rays from outside, and the data can again be written therein. Therefore, this is an extremely useful memory device in the development of new data processing techniques and the like.
In conventional write-in control of the PROM, the data in each word unit is written into a memory array and the program operation is repeated serially on each word. Therefore, as the memory capacity for one chip becomes greater the time required for writing in becomes longer to the same extent, since data can be written in only one word unit. For instance in the case of N channel type MOS of the floating gate type the data write-in time in a unit memory cell is no more than 100 msec, but the program time required for the whole of a memory array with for instance a 1K word is 100 sec. As a method of making the program time shorter, an easy way is the procedure of making the number of bits for one word (ordinarily about 8 bits) extremely large, whereby the number of words is made smaller while the memory capacity remains the same. But if the number of bits for a unit word in the memory array is increased, the data lines become more numerous and therefore the number of external terminals for input and output increases, the merit of LSI (large scale integrated circuits) is impaired, and also trouble occurs at the interfaces with CPU and other external equipment.
Accordingly, with the known PROM the read-out time is short (400 to 500 nsec) and it has satisfactory characteristics as regards use. But on the other hand the time for writing in specific data, programs and the like is very long, and this fact has become a source of difficulty more particularly in checking the characteristics of mass-produced PROM, and in cases in which data needs to be written into a large number of PROMs in a short time, and the like.